Generally, in the production of a display device, a thin-film transistor (TFT) plays a very important role. A pixel capacitor of the display device is rapidly charged by using an on-state of the thin-film transistor, and a voltage of the pixel capacitor is sustained by using an off-state of the thin-film transistor, so that unification of rapid response and good storage is realized. The thin-film transistor as a nonlinear switch is widely applied to fields such as a large-area liquid-crystal display and a contact type image sensor, due to a very high on-state current (Ion) to off-state current (Ioff) ratio and a steep transfer characteristic.
At present, a specific structure of a conventional bottom-gate unstack type amorphous silicon thin-film transistor is shown in FIG. 1, which comprises a substrate 01, a gate electrode 02 disposed on the substrate 01, an active layer 03 disposed on the gate electrode 02 and insulated from the gate electrode 02, as well as a source electrode 04 and a drain electrode 05 which are oppositely arranged and electrically connected with the active layer 03 respectively. When a current is applied to the gate electrode 02 by a circuit mounted in the substrate 01, a current loaded to the source electrode 04 is transmitted to the drain electrode 05 through the active layer 03, thereby driving a pixel unit of the display device to display an image. A gate insulating layer 06 is disposed between the gate electrode 02 and the active layer 03, and a passivation layer 07 is disposed over the source electrode 04 and the drain electrode 05. The gate insulating layer 06 is generally made of an a-SiNx thin film, the active layer 03 is generally made of an a-Si:H thin film, the passivation layer 07 is generally made of an a-SiNx thin film, and the gate electrode 02, the source electrode 04 and the drain electrode 05 are generally made of a metal chromium material. In order to improve contact characteristics between the source electrode 04 and the a-Si:H thin film as well as between the drain electrode 05 and the a-Si:H thin film, thin n+ type a-Si:H thin films are inserted therebetween to serve as ohmic contact layers 08.
The a-Si:H thin film serving as the active layer of the thin-film transistor is a material with a good light sensitive characteristic. However, under a backlight illumination condition, a resistance value of the active layer per se may change, so that the off-state current of the thin-film transistor is increased by 2-3 orders of magnitudes. In this way, the on-state current to off-state current ratio of the thin-film transistor is greatly reduced, and the image display quality of a liquid-crystal display is seriously affected.
Consequently, a technical problem to be solved by those skilled in the art is how to increase the on-state current to off-state current ratio of the thin-film transistor under the backlight illumination condition.